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Jakarta, Indonesia (CNN)Three days after ISIS militants carried out a deadly terror attack in broad daylight, Indonesians poured out by the thousands onto the streets of this tropical city for an exuberant day of recreation.

A stream of families walked, jogged, cycled, munched street food and enjoyed live music performances on Sunday morning. The festive scene took place within yards of the Starbucks that ISIS members attacked with suicide bombs and grenades on January 14.

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For years every weekend, this teeming, steamy city closes vehicular traffic on several main boulevards as part of an initiative called "Car Free Day." But there was a spirit of defiance among many residents of the capital this weekend.

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"This Sunday I'm quite happy, because look, everybody's here," said Rainier Daulay, an Indonesian hotel owner who took a break from exercising on his bicycle to smoke a cigarette.

"We don't care about terrorism."

He stood near a police traffic control booth that had been attacked by ISIS militants on Thursday. The battle-scarred structure was now decorated with the colors of the Indonesian flag as well as slogans that translated as "Brave Indonesia, Peaceful Indonesia."

In front of the Starbucks, still gutted and burned on the inside from the attack, men dressed in traditional costumes and skull and feather head-dresses danced with spears to the beat of a drum in a show of Indonesian patriotism.

New threat

Thursday's attack did not approach the scale of carnage caused by the 2002 al Qaeda-linked bombing on the Indonesian island of Bali, which killed more than 200 people.

In fact, counter terror experts here say ISIS only attracts a fringe minority of support in the world's most populous Muslim country.

Photos:Deadly explosions, gunfight in Jakarta

Photos:Deadly explosions, gunfight in Jakarta

Employees inspect the damage on January 15, 2016 at the Starbucks cafe where deadly attacks occurred the day before in central Jakarta, Indonesia. ISIS claimed responsibility for the attack in an official statement posted online.

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Photos:Deadly explosions, gunfight in Jakarta

Students light candles to express solidarity for those affected by the deadly attack in Jakarta during a vigil on January 14 in Surabaya, East Java, Indonesia.

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Photos:Deadly explosions, gunfight in Jakarta

A bouquet of flowers is placed in front of an explosion site on January 14 in Jakarta.

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Photos:Deadly explosions, gunfight in Jakarta

A policeman stands guard in front of a blast site in central Jakarta after a series of explosions rocked the heart of the Indonesian capital on Thursday, January 14.

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Photos:Deadly explosions, gunfight in Jakarta

Police and ambulances arrive in front of the Sarinah shopping center, next to a police station targeted by the attacks, on January 14.

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Photos:Deadly explosions, gunfight in Jakarta

Armed police officers keep watch outside a damaged Starbucks cafe after an attack in Jakarta on January 14.

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Photos:Deadly explosions, gunfight in Jakarta

Police officers stand guard outside a Starbucks cafe in Jakarta's bustling shopping area, near the site of an explosion, on January 14.

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Photos:Deadly explosions, gunfight in Jakarta

Police officers are deployed near the site where a series of blasts rocked downtown Jakarta on January 14.

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Photos:Deadly explosions, gunfight in Jakarta

Indonesian police take position and aim their weapons as they pursue suspects outside a cafe after explosions went off in Jakarta on January 14.

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Photos:Deadly explosions, gunfight in Jakarta

Smoke billows from an explosion in downtown Jakarta on January 14.

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Photos:Deadly explosions, gunfight in Jakarta

Plainclothes police officers aim their guns at suspects outside a cafe after a series of blasts hit the center of Jakarta on January 14.

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Photos:Deadly explosions, gunfight in Jakarta

Smoke rises following a series of blasts outside a shopping center in Jakarta on January 14.

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Photos:Deadly explosions, gunfight in Jakarta

Indonesian police take position behind a vehicle as they pursue suspects after a series of blasts hit the center of Jakarta on January 14.

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Photos:Deadly explosions, gunfight in Jakarta

A policeman fires his handgun towards suspects outside a cafe on January 14.

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Photos:Deadly explosions, gunfight in Jakarta

A body lies in the middle of the street near a damaged police post on January 14.

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Photos:Deadly explosions, gunfight in Jakarta

Police hide behind vehicles during an exchange of gunfire with suspects hiding near a Starbucks cafe when another blast took place in Jakarta on January 14.

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Photos:Deadly explosions, gunfight in Jakarta

Police appear to chase suspects thought to be hiding at a cafe on January 14.

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And yet, law enforcement authorities warn ISIS represents a new generation of terrorists that could be more lethal than their al Qaeda predecessors.

"ISIS is more dangerous in the case of Indonesia...because they perceive Indonesia as not Dawla Islamiya, not an Islamic country," said Jakarta police chief Tito Karnavian in an interview with CNN.

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At that time, police arrested Bahrun Naim for possession of ammunition. He was sentenced to more than two years in prison.

Abu Ulya said that in prison, "Bahrun Naim became more militant ... he got closer to other jihadis."

Abu Ulya insisted that it wasn't until January 2015 that he knew Bahrun Naim had suddenly moved his family thousands of miles away to Syria. The radical had joined ISIS in Raqqa, the Syrian city that became the capital of the group's self-declared caliphate.

"His vision is to join, to unite all ISIS-supporting elements in southeast Asia: Indonesia, Malaysia and the Philippines," said Karnavian, the Jakarta police chief.

Communication from ISIS stronghold

Abu Ulya held up a Lenovo smartphone.

There, in a text messaging application called Telegram that he said was favored by supporters of ISIS in Indonesia, was a long series of messages exchanged with someone identified as Syam BN.

Syam is an Indonesian transliteration for the Arabic name for the Levant region of the eastern Mediterranean, which includes Syria.

Abu Ulya said several months after Bahrun Naim reached Syria, he contacted him via Telegram.

One of the final messages he received from the militant was dated December 3, 2015. Translated from the Bahasa Indonesian language it said "sorry sorry I was influenced by the boys." The message ended with two smiley-faced emoticons.

An undated photo of Bahrun Naim posted on his Telegram profile shows him dressed in a leather jacket, leaning against a wall decorated with Arabic flyers, holding a Kalashnikov assault rifle and wearing a Middle Eastern kaffiyeh scarf. The architecture of the buildings behind him is similar to structures one would see in many Syrian and Iraqi towns.

Since ISIS announced the creation of a caliphate around Raqqa in 2014, the region has served as a magnet for jihadis from around the world.

One ISIS-recruiting video shows a small group of men walking along a lake carrying weapons and an ISIS flag.

"We are your brothers from Indonesia who have come to join the Islamic state," says a man identified as Abu Muhammad al Indonesi. Speaking in Bahasa Indonesia, he urges fellow Muslims in Indonesia to drop familial and professional obligations to emigrate to Syria.

In a separate video, Indonesians demonstrate how they celebrate Eid al Adha, or the Feast of the Sacrifice, in Raqqa. And they repeat the appeal for Indonesians to make the pilgrimage to the ISIS stronghold.

Communicating via Telegram, Abu Ulya said he learned that Bahrun Naim took two wives and two children with him to Syria.

In fact, at Bahrun Naim's request, Abu Ulya said he informed the parents of the second wife that their daughter had gone to Syria.

"The parents are so sad losing her," he said.

In November, Abu Ulya learned that Bahrun Naim had been injured in Syria.

"He told me he was experimenting with chemicals and there was an accident," Abu Ulya said.

"Because of the accident, in his own words he became an 'office boy.'"

Around 14 weeks ago, someone identified as Bahrun Naim began posting blogs on a Google-plus account.

The articles include hand-drawn instructions on how to make an explosive belt, as well as lessons on making remote-controlled bombs and detonators.

Abu Ulya described Bahrun Naim as an "urban guerilla." He disputed the Indonesian government's claims, however, that his "little brother" was the leader of the ISIS movement in Indonesia, arguing that the 32-year old was too young to hold such a position.

However, several scholars of Indonesia's radical Islamist movements agree that with his social media profile and his presence in Syria, Bahrun Naim has become an inspiring figure for other young would-be supporters of ISIS on the Indonesian archipelago.

"Many people respect him even though he's very young," said Al Chaidar, a lecturer at Malikussaleh University in Indonesia's Aceh region.

"We can see the new role of a new generation of the jihadi movement," Al Chaidar added.

Security investigation

Last December, Indonesian police ramped up security before Christmas after intercepting communications that ISIS was planning "a concert." They interpreted that to be a codeword for an attack.

Abu Ulya told CNN that in November 2015 he received a similar message from his old friend in Syria.

"He once told me that he wanted to make a concert," Abu Ulya said. "The date of the concert is not fixed yet, but he told me just to wait."

Abu Ulya said it was far too early to conclude that Bahrun Naim planned the January 14 terror attack.

The militant's younger brother Dahlan told Indonesian media last weekend not to jump to conclusions about his sibling's alleged role in the suicide bombing.

"Regarding those allegations against Bahrun, I think we should just let the law take its course," said Dahlan Naim.

Journalist Mohammad Eyad Kourdi contributed to this report from the Syrian-Turkish border.